Generally, the operating frequency of a piezoelectric crystal will vary when exposed to different temperatures. This is called the frequency-temperature (FT) response curve of the crystal. The function of an oven controlled crystal oscillator (OCXO) is to minimize this frequency change over temperature by maintaining the crystal within a stable thermal environment. Additionally, the normal temperature dependence of some components of the oscillator's amplifier and feedback networks will change loop reactance, and thereby the output frequency. These components, along with the crystal, are called the critical components. Typically, the critical components are kept within the guarded temperature range of an oven so as to maintain the critical components at a relatively constant temperature. However, in practice, oven temperatures are never perfectly constant. They will vary over a small range. Therefore, it is best to operate the oven at a nominal temperature where the FT response of the crystal is relatively flat. In this way, if the oven temperature varies slightly, the crystal frequency will not change substantially. Crystal FT responses are nominally flat at minimum or maximum (turning) points on their FT curves. Typically, the crystals chosen for OCXO applications have a minimum or a maximum turning point somewhat above a normal ambient operating temperature so as to enable the use of a heating element to maintain a constant temperature, but not so high as to cause poor long term stability (aging) of the crystal, reliability impact on the associated electronics, or excessive power consumption. The most popular crystals for this type of application are AT and SC-cut quartz which have a turning point in their FT response somewhat above the highest expected operating ambient temperature.
Typically, OCXOs are provided with separate fixed or variable components for both tuning a crystal oscillator onto a desired frequency and setting an oven at a desired temperature. Prior art OCXOs provide external access to a variable capacitor to adjust (warp) the crystal oscillator onto a desired nominal frequency. However, the non-hermetic opening provided for this external access adversely affects temperature stability within the temperature controlled volume of the oven by allowing convective heat transfer to the environment. Further, the opening can lead to degradation of insulation or corrosion of internal components when the OCXO is operated in humid or otherwise hostile environments. In addition, OCXOs using analog oven circuits have required the manual insertion of components to match oven temperature to a crystal's turning point. Alternatively, OCXOs using digital oven circuits have required the use of additional A/D circuits to convert a temperature sensor signal into a digital form.
In operation, an oven is individually set at a turning point temperature for each particular crystal. To accomplish this, crystals undergo preliminary testing to determine the temperatures of their turning points. However, as the OCXO is assembled or sealed or the crystals are subjected to mounting or thermal cycling stresses, the turning points or frequency of the crystal may shift. Also, the thermal paths in the OCXO will typically change when the package is sealed. This changes the temperature gradients from the critical components to the temperature sensor. In addition, convection current variations, burn-in testing, and quality control testing may cause the turning points or frequency of the crystal to shift. This causes a problem because the package of an OCXO does not easily allow for re-adjustment of the oven temperature. Therefore, any shifts are difficult to recompensate without opening the OCXO to substitute or adjust components. Afterwards, there is no guarantee that the crystal or package thermal gradients may not shift again during reseating.
Another problem with the present use of manually adjusted warp components is that, although they generally have a significant adjustment range to compensate for a limited frequency accuracy of the crystal, they are not always made available to the end user. This may occur when a customer requires a completely hermetic package, thereby sealing off the warp components from external access. Without this access the working life of the OCXO is limited. If the warp adjustment were available to the end user, the working life of the OCXO could be extended.
There is a need for a temperature-controlled frequency source having a temperature control component and a crystal oscillator that: are sealed from the outside environment; do not need separate fixed or variable components for adjustment; can be adjusted before and after assembly of the source or during use in the field; do not require multiple adjustments to be properly set; and can be electrically adjusted without having to mechanically or thermally disturb the internal circuitry of the frequency source. In addition, it is desirable to provide a low cost, small sized frequency source that uses a reduced number of components, and does not require the use of a separate oven chamber.